Semiconducting (conjugated) polymers have been studied as electroluminescent materials for use in light emitting displays since the early 1990's. Such emissive polymer displays offer a number of advantages, including high brightness at low operating voltage, low weight, thin profile and low power consumption. The relatively simple processing enabled by the use of soluble semiconducting polymers provides a pathway to low cost, high volume fabrication.
The requirements of a conjugated polymer for use in high efficiency, stable light emitting devices include high photoluminescence (PL) efficiency, good film forming ability, good thermal stability, and balanced carrier injection and transport.
High photoluminescence (PL) efficiency: In an efficient luminescent polymer medium, radiative recombination is favored over non-radiative recombination. PL efficiencies in excess of 10% are preferred; PL efficiencies in excess of 25% are still more preferred; and PL efficiencies in excess of 50% are still more preferred.
Good film forming ability: The emissive polymer should have high molecular weight; polymers which can be cast into high quality pin-hole free films by processing from solution, for example by spin-casting, are preferred. Solution processability can be achieved by introducing flexible side chains such as alkyl or alkoxy groups onto the backbone of the conjugated polymer. The polymerization chemistry should be chosen with the goal of achieving high molecular weight.
Good thermal stability: To ensure good thermal stability, the polymer should be designed to have a high glass transition temperature (Tg).
Balanced carrier injection and transport: High efficiency light emitting diodes (LEDs) require balanced carrier injection and transport. Ideally, the injection of holes at the anode and electrons at the cathode should be equal, and the transport mobility of electrons and holes in the polymer should be equal. For these conditions to be realized, certain electron withdrawing (and/or electron donating) groups should be incorporated into the polymer to improve electron (and/or hole) injection.
Several classes of luminescent polymers have been disclosed in the art heretofore. These include, for example, poly[1,4-phenylene vinylene] (or “PPV”) (J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539 (1990)); soluble derivatives of PPV, such as (2-methoxy-5-(2′-ethylhexyloxy)-PPV (or “MEH-PPV”) (U.S. Pat. No. 5,189,136); aryl-substituted-PPV (WO98/27136); and PPV copolymers (EP-544795, WO98/04610, H. Becker, H. Spreitzer, Y. Cao, Adv. Mater. 12(1), 42 (2000)). Soluble derivatives of polythiophene are also known in the art, e.g., the poly(3-alkylthiophenes) (D. Braun, G. Gustafssom, D. Mcbranch, J. Appl. Phys. 72, 564 (1992)). The photoluminescent spectra of these polymers typically fall in the visible spectral region with colors ranging from green to red. Considerable progress have been made toward using these materials in light emitting displays with lifetimes sufficient for commercial products (U.S. Pat. No. 5,798,170, I. Park, Y. Cao and C. Y. Yang, J. Appl. Phys. 85(4), 2441 (1999)).
For full-color display applications, blue-emitting materials are required. The search for an efficient blue-emitting electroluminescent (EL) polymer with long EL lifetime is ongoing. Among the fully conjugated and partially conjugated polymers that have been designed and synthesized, only a limited number showed promise for stable blue EL emission. Polymers and oligomers of alkyl-substituted fluorene have been disclosed by several groups. Fukuda et al. prepared 9-alkylated polyfluorene by treating the monomer with a large excess of oxidizing metal salt such as ferric chloride (Fukuda et al., Japanese J. Appl. Phys. 28, 1433-1435 (1989)). This process suffered from crosslinking and mislinking reactions during polymerization and resulted in a low molecular weight polymer with high polydispersity. Pei et al. disclosed polyfluorenes containing polar-alkyl substituents using dibromofluorene as a monomer (U.S. Pat. No. 5,900,327). Unfortunately, the molecular weight of the polymer obtained using this method is low. E. P. Woo et al. used the Suzuki coupling reaction for obtaining polymers and oligomers of alkyl-substituted fluorene (U.S. Pat. No. 5,777,070 and U.S. Pat. No. 5,708,130). Although relatively high EL efficiencies have been achieved for green and red polyfluorene-based LED devices with promising lifetimes, similar results for blue LEDs have not been reported (W. Wu et al., Meeting of the Society for Information Display, 1999, San Diego, Calif.).
As a result of the electron-withdrawing character of the 1,3,4-oxadiazole ring, certain low molecular weight aromatic 1,3,4-oxadiazole derivatives facilitate the injection and transport of electrons. These oxadiazole derivatives have been used in organic LED devices fabricated by the vapor deposition method (C. Adachi. T. Tsutsui, and T. Saito, Appl. Phys. Lett. 56, 799 (1990); C. Adachi. T. Tsutsui, and T. Saito, Appl. Phys. Lett. 57, 531 (1990); U.S. Pat. No. 5,656,401). However, these liquid crystalline oxadiazoles exhibited poor thermal stability. The exposure of the devices to elevated temperatures caused recrystallization or aggregation, and hence resulted in short device lifetimes (K. Naito, A. Miura, J. Phys. Chem. 97, 6240 (1993)).
The oxadiazole group has also been incorporated into the main chain of various polymers (Q. Pei, Y. Yang, Chem. Mater. 7, 1586 (1995); B. Schulz, Y. Kaminorz, L. Brehmer, Synth. Met. 84(1-3), 449 (1997); and Z. H. Peng, Z. N. Bao, M. E. Galvin, Adv. Mater. 10(9), 680-684 (1998)). The polymers reported in this group were either only partially conjugated, or, when fully conjugated, had low molecular weights. The EL quantum efficiencies were poor for devices where one of these polymers was used as the emissive layer.
Polymers with the oxadiazole group as a pendant group suffered similar problems (M. Greczmiel, P. Pösch, H.-W. Schmidt, P. Strohriegl, Macromol. Symp. 102, 371 (1996), Z. Bao, Z. Peng, M. E. Galvin, and E. A. Chandross, Chem. Mater. 10(5), 1201 (1998)).
Thus, there is a need for blue-emitting conjugated polymers, and more generally, visible light emitting polymers that exhibit high PL efficiency, good film forming ability, good thermal stability, and that have electronic structures consistent with balanced carrier injection and transport.